cmd/geth, cmd/utils: improve input handling

These changes make prompting behave consistently on all platforms:

* The input buffer is now global.
  Buffering was previously set up for each prompt, which can cause weird
  behaviour, e.g. when running "geth account update <input.txt" where
  input.txt contains three lines. In this case, the first password
  prompt would fill up the buffer with all lines and then use only the
  first one.

* Print the "unsupported terminal" warning only once.
  Now that stdin prompting has global state, we can use it to track
  the warning there.

* Work around small liner issues, particularly on Windows.
  Prompting didn't work under most of the third-party terminal emulators
  on Windows because liner assumes line editing is always available.

cmd/geth/main.go

@@ -373,6 +373,7 @@ JavaScript API. See https://github.com/ethereum/go-ethereum/wiki/Javascipt-Conso
 	app.After = func(ctx *cli.Context) error {
 		logger.Flush()
 		debug.Exit()
+		utils.Stdin.Close() // Resets terminal mode.
 		return nil
 	}
 }
@@ -595,12 +596,12 @@ func getPassPhrase(prompt string, confirmation bool, i int, passwords []string)
 	}
 	// Otherwise prompt the user for the password
 	fmt.Println(prompt)
-	password, err := utils.PromptPassword("Passphrase: ", true)
+	password, err := utils.Stdin.PasswordPrompt("Passphrase: ")
 	if err != nil {
 		utils.Fatalf("Failed to read passphrase: %v", err)
 	}
 	if confirmation {
-		confirm, err := utils.PromptPassword("Repeat passphrase: ", false)
+		confirm, err := utils.Stdin.PasswordPrompt("Repeat passphrase: ")
 		if err != nil {
 			utils.Fatalf("Failed to read passphrase confirmation: %v", err)
 		}